The present invention relates to a weighing device having a plurality of digital weighing cells, each comprising                a sensor connected to a force introducing device and configured to generate a digital measurement value corresponding to an imparted force at a measurement time point,        a data processing device configured to convert the digital measurement values to transmission units for transmission over a data communications line, and to transmit the transmission units to a central control unit connected to each of the weighing cells for analysis of the transmission units,wherein the central control unit calculates a digital combination value based on the transmission units originating from each of the various weighing cells, and wherein the combination value represents a weight force which is exerted upon the weighing device at a weighing time point.        
The invention also relates to a method for determining a digital combination value which represents a weight force which is exerted upon the weighing device at a weighing time point, comprising:                requesting, with a central control unit, a digital measurement value from each of a plurality of weighing cells, each of the weighing cells having (i) a sensor connected to a force introducing device, and (ii) a data processing device,        in each of the plurality of weighing cells:                    generating, with the sensor, a digital measurement value corresponding respectively to an imparted force at a measurement time point,            converting, with the data processing device, the digital measurement value to a transmission unit configured to be transmitted over a data communications line, and            transmitting, with the data processing device, the transmission unit to the central control unit for analysis, and                        calculating, with the central control unit, the digital combination value based on the transmission units originating from the various weighing cells.        
Weighing devices and methods of this type are discussed in the article by Rauchschwalbe, U.: “Digitale Wägezellen: Innovation oder Marketing Gag?”, wdm: wägen, dosieren, mischen, issue 1, March 2007.
Industrial weighing devices typically comprise a plurality of weighing cells which support a weighing platform at different positions. A weight force acting on a weighing platform is generated via the force introducing device of each weighing cell to the sensor thereof, which generates a partial measurement value. A suitable combination of the partial measurement values results in the combination value which represents the overall weight force that acts on the weighing platform.
In many known devices, the sensors are analog sensors which generate an analog electrical voltage at their output. Parallel connection of all the analog weighing cells supporting the weighing platform corresponds to an addition of the voltages to a combination voltage. The analog combination voltage is typically digitized in devices of this type and is passed on as a series of digital values for further processing or analysis.
Replacing such analog weighing cells with digital weighing cells which, in view of the modularity of the system and the avoidance of error-prone analog lines, would be desirable. However, such replacement leads, as discussed in detail in the aforementioned article, to various difficulties. Digital weighing cells generate digital measurement values at predetermined measurement time points. The sensor of a digital weighing cell typically also comprises one or more analog force transducers, for example, strain gauges, which generate an analog voltage that is converted by an analog-to-digital converter (ADC) of the sensor into a sequence of individual digital measurement values, wherein each digital measurement value represents the force applied to the force transducer at the measurement time point. In order to combine the digital measurement values of a plurality of weighing cells, the individual values must be sent via a data communications line to a central control unit and further processed there. For this purpose, conversion of the measurement values to transmission units which can be transmitted via the data communications line is required. A widely used standard interface for digital weighing cells is the serial RS 485 interface, by which the information relating to the measurement values is transmitted according to a standardized protocol. Other interfaces and bus systems are also possible, wherein the conversion of the measurement values to transmission units must always be carried out to suit the communications network used.
As distinct from the analog case where the combining of measurement values is achieved with the parallel connection of the cells without any time delay, in the case of digital weighing cells, the chronological matching of the values to be combined is particularly significant. Typically, therefore, the digital weighing cells operate in a synchronized manner wherein the synchronization is brought about for example with a synchronizing pulse from the central control unit, requesting all the weighing cells to generate a digital measurement value simultaneously and then sequentially requesting the measurement values which have been temporarily stored in the individual measuring cells. The sequentially requested measurement values are then processed in the central control unit to form a combination value which is assigned to the synchronization time point. In this way, a sequence of individual chronologically allocated combination values is formed in the central control unit, representing a sampled progression of the weight force exerted upon the weighing platform. A disadvantage of this system is the substantial length of the interval between individual combination values, which increases as the number of weighing cells to be taken into account rises. This is disadvantageous for dynamic weighing processes, just as it is for dosing procedures in which precise and rapid predictability of the progression of the weight force exerted upon the weighing devices is of essential importance.
Although dispensing with the synchronization of the measurement time points can lead to acceleration of the speed of the combination value updating, which is then dependent only on the speed of the data communications line and the central control device, each combination value is based on individual measurement values which have been recorded at different measurement time points, which leads, particularly in the case of dynamic processes, to significant inaccuracy in the measurement result.
The aforementioned article proposes an at least partial return to analog weighing cells. In order to avoid errors it is proposed, in particular, to pass the analog signals via a short analog communications line to a multi-channel ADC which digitizes the analog input signals on all channels synchronously and combines the digital values generated. A disadvantage of this concept is the loss of modularity gained by introducing the digital weighing cells and the reintroduction of analog line sections with their known error and calibration problems.